The tendency in recent years towards higher mounting densities and larger capacities of large-scale integrated (LSI) circuits are further reducing the circuit line widths needed for semiconductor devices. Fabrication of semiconductor devices involves the use of photomasks or reticles (hereinafter, referred to collectively as masks) each having circuit patterns formed thereon. The circuit patterns on a mask are photolithographically transferred on to a wafer using a reduction projection exposure apparatus, often called a stepper, whereby the circuit patterns are formed on the wafer. An electron beam writing apparatus capable of writing fine patterns is used to manufacture the masks used to transfer the fine circuit patterns onto the wafer. Developing a laser beam writing apparatus for writing patterns with laser beams has also been attempted. An electron beam writing apparatus is also used for writing circuit patterns directly onto the wafer.
An electron beam writing apparatus has inherently superior resolution, and can ensure greater depth of focus, thus allowing control over size fluctuations even with a high difference in level. Japanese Patent Application Laid-Open No. Sho 61-34936 discloses a method for manufacturing a semiconductor integrated circuit device using an electron beam writing apparatus.
When a pattern is written on a mask using such an electron beam writing apparatus, it is necessary to measure the surface height of the mask before writing to avoid displacement of a writing position or defocusing of an electron beam. If the surface of the mask is perfectly flat, a desired pattern can be written by measuring the surface height of the mask, at any one point on the mask, and focusing the electron beam based on this surface height. However, in fact, the surface of the mask is not perfectly flat and is slightly deformed. Therefore, when the electron beam is always focused based on a surface height measured at only one point on the mask, defocusing occurs in some portions on the mask.
In order to solve such a problem, Japanese Patent Application Laid-Open No. Sho 61-34936 discloses a method for correcting defocusing caused by deformation of a mask. According to this method, the surface height of a mask is first measured at several points on the mask to obtain a plurality of quadratic equations. Then, the coefficients of these quadratic equations are determined by the least squares method, and a height correction equation using the obtained coefficients is used to perform focus correction and deflection gain correction.
In addition, Japanese Patent Application Laid-Open No. Hei 5-47648 discloses the height in several points outside the exposure territory of the mask is measured, the method of setting the height of lighting position from the value which is obtained is disclosed.
However, in the correction method described in the above patent documents, the height of the mask is calculated by interpolation using measurement results measured at several points on the surface of the mask. Therefore, the surface shape of the actual mask can be approximated but the exact surface shape of the mask cannot be determined.
Further, in order to improve the throughput of an electron beam writing apparatus, writing time is reduced by increasing the moving speed of a stage in a region where a pattern density is low. When the moving speed of the stage is changed as described above, the stage undergoes acceleration at the beginning of the region and then undergoes deceleration at the end of the region. At this time, inertial force is exerted on a mask by acceleration and deceleration of the stage, and therefore the surface height of the mask varies and becomes different from height data determined before writing.
On the one hand, if it is possible to measure the height of the mask surface when writing, it is therefore possible to solve the above-mentioned problem. But, in this case, the following kind of problem occurs.
When a mask is irradiated with an electron beam, a resist formed on a light-blocking layer formed on a mask substrate with the use of a conductive material such as chromium (Cr) becomes charged. When the mask is kept irradiated in such a state, the path of the electron beam is deflected by an electric field created by the charged mask, which makes it impossible to perform writing to a desired position. In order to solve such a problem, a charged layer is grounded. For example, a support mechanism is provided so as to surround a mask placed on a stage, and a substrate cover is provided on the support mechanism. The substrate cover is grounded by connecting the support mechanism to ground, and therefore electrons scattering near the peripheral portion of the mask are captured by the substrate cover so that the peripheral portion of the mask is prevented from becoming charged.
However, when such a substrate cover is used to measure the surface height of a mask in real time during writing, height measurement cannot be performed in some regions. This results from that height measurement is performed by detecting the reflection of light emitted toward a point near the irradiation region of an electron beam. That is, as described above, the substrate cover is placed over the mask during writing. Therefore, light emitted toward the mask for height measurement is blocked by the substrate cover in a case where the surface height of the mask needs to be measured near the substrate cover. In this case, light reflected from the surface of the mask cannot be detected, which makes it impossible to perform height measurement.
Even when height measurement can be performed, the following problem may occur. Height measurement data acquired during writing is sent from a height measuring unit to a writing control unit of an electron beam writing apparatus, and then an electron beam optical unit is adjusted based on this data. However, for example, if errors occur during data communication between the height measuring unit and the control unit, the height measurement data is not sent to the control unit. In this case, the control unit fails to acquire data as in a case where height measurement cannot be performed.
The present invention has been made with the above taken into account. That is, an object of the invention is to provide a charged-particle beam writing apparatus measuring exactly the surface shape of a sample and then writing high accuracy.
These and other advantages of the present invention will become clear in view of the following description.